The internal pressure measurement of blood vessels, in particular the internal pressure measurement in lung arteries plays a decisive role in modern medicine. The pressure measurement of pulmonary arteries is an important quantity for lung and heart function assessment. This pressure measurement in pulmonary arteries is used for the diagnosis of shock states, primary pulmonary hypertension, pulmonary embolus or other severe left ventricular failure.
Unfortunately, the pulmonary artery pressure can only be measured reliably using an expensive catheter procedure. In medicine this procedure is usually referred to as pulmonary artery catheterization or referred to as Swan-Ganz catheter.
According to this procedure the catheter is introduced through a large vein, often the internal jugular, subclavian or femoral veins. From this entry side, it is threaded through the right atrium of the heart, the right ventricle and subsequently into the pulmonary artery. The standard pulmonary artery catheter has two lumens and is equipped with an inflatable balloon at its tip, which facilitates its placement into the pulmonary artery through the flow of blood. The balloon, when inflated, causes the catheter to “wedge” in a small pulmonary blood vessel. So wedged, the catheter can provide a measurement of the pressure in the left atrium of the heart.
Unfortunately, this procedure is due to its invasive complicated character not at all without risk, and complications can be live threatening. It can lead to arrhythmias, rupture of the pulmonary artery, thrombosis, infection, pneumothorax, bleeding, and other problems. Therefore many physicians minimize its use. Of course, other indirect methods like arterial gas levels and ultrasound are known to measure the pressure in the pulmonary arteries, but they just give a valuable insight into the disease, but no definitive answer. Reliable non-invasive methods have so far not been found.
Magnetic Particle Imaging (MPI) is an emerging medical imaging modality. The first versions of MPI were two-dimensional in that they produced two-dimensional images. Current and future versions will be three-dimensional (3D). A time-dependent, or 4D, image of a non-static object can be created by combining a temporal sequence of 3D images to a movie, provided the object does not significantly change during the data acquisition for a single 3D image.
MPI is a reconstructive imaging method, like Computed Tomography (CT) or Magnetic Resonance Imaging (MRI). Accordingly, an MP image of an object's volume of interest is generated in two steps. The first step, referred to as data acquisition, is performed using an MPI scanner. The MPI scanner has means to generate a static magnetic gradient field, called “selection field”, which has a single field free point (FFP) at the isocenter of the scanner. In addition, the scanner has means to generate a time-dependent, spatially nearly homogeneous magnetic field. Actually, this field is obtained by superposing a rapidly changing field with a small amplitude, called “drive field”, and a slowly varying field with a large amplitude, called “focus field”. By adding the time-dependent drive and focus fields to the static selection field, the FFP may be moved along a predetermined FFP trajectory throughout a volume of scanning surrounding the isocenter. The scanner also has an arrangement of one or more, e.g. three, receive coils and can record any voltages induced in these coils. For the data acquisition, the object to be imaged is placed in the scanner such that the object's volume of interest is enclosed by the scanner's field of view, which is a subset of the volume of scanning.
The object must contain magnetic nanoparticles; if the object is an animal or a patient, a contrast agent containing such particles is administered to the animal or patient prior to and/or during the scan. During the data acquisition, the MPI scanner steers the FFP along a deliberately chosen trajectory that traces out the volume of scanning, or at least the field of view. The magnetic nanoparticles within the object experience a changing magnetic field and respond by changing their magnetization. The changing magnetization of the nanoparticles induces a time dependent voltage in each of the receive coils. This voltage is sampled in a receiver associated with the receive coil. The samples output by the receivers are recorded and constitute the acquired data. The parameters that control the details of the data acquisition make up the scan protocol.
In the second step of the image generation, referred to as image reconstruction, the image is computed, or reconstructed, from the data acquired in the first step. The image is a discrete 3D array of data that represents a sampled approximation to the position-dependent concentration of the magnetic nanoparticles in the field of view. The reconstruction is generally performed by a computer, which executes a suitable computer program. Computer and computer program realize a reconstruction algorithm. The reconstruction algorithm is based on a mathematical model of the data acquisition. As with all reconstructive imaging methods, this model is an integral operator that acts on the acquired data; the reconstruction algorithm tries to undo, to the extent possible, the action of the model.
Such an MPI apparatus and method have the advantage that they can be used to examine arbitrary examination objects—e.g. human bodies—in a non-destructive manner and without causing any damage and with a high spatial resolution, both close to the surface and remote from the surface of the examination object. Such an arrangement and method are generally known and are first described in DE 101 51 778 A1 and in Gleich, B. and Weizenecker, J. (2005), “Tomographic imaging using the nonlinear response of magnetic particles” in nature, vol. 435, pp. 1214-1217. The arrangement and method for magnetic particle imaging (MPI) described in that publication take advantage of the non-linear magnetization curve of small magnetic particles.